Gastrulation - Evidence for Apical Constriction

Evidence for apical constriction comes from several sources. Here the
vegetal plate in L. pictus has been stained with phalloidin to reveal actin
microfilaments. The arrows point to two regions of constriction that make
local "kinks" in the vegetal plate. Robert Burke and colleagues
at the University of Victoria were the first to show that a dense localization
of actin is present in these apically constricted. The picture below shows
an example of this from the species they work on, S. purpuratus. (the
micrograph was generated in our laboratory).

The picture below is a scanning electron micrograph of a comparable vegetal
plate in L. pictus, which also shows the kinks at locations of bottle cells formation. Since these
are sagittal sections through the vegetal plate, if these cells formed all
the way around the vegetal plate, we would expect a ring of such cells if
we could look at the surface of the vegetal plate from the bottom (i.e.,
the vegetal pole). The next page
shows how we can do this...